Biodiesel is a nontoxic and biodegradable fuel that can be used in conventional diesel engines. As a fuel, biodiesel is a renewable alternative to standard petroleum-based diesel fuels. Typically, biodiesel is produced from oils and sources of free fatty acids such as, for example, vegetable oil, animal fat and waste type greases. Biodiesel is usually obtained through an esterification or, preferably, transesterification type process in which triglycerides derived from oil or fat are reacted with alcohol in the presence of a catalyst. In general, the form of biodiesel yielded via a transesterification process can depend on the types of alcohols or solvents employed. For example, often methanol is employed in a transesterification process to obtain a fatty acid methyl ester biodiesel. Alternatively, ethanol can be used in a transesterification process to yield a fatty acid ethyl ester biodiesel. Such biodiesel type esters of fatty acids are commonly referred to as mono-alkyl esters or monoesters. An esterification process can also employ cosolvents such as, for example, methanol and tetrahydrofuran (THF) in order to yield a fatty acid ester biodiesel.
Mono-alkyl esters of fatty acids comprise those fatty acids associated with the oils or fats used in the transesterification process. For naturally occurring oils and fats, the fatty acids of a fatty acid ester biodiesel include, for example, linoleic, stearic, palmitic or oleic acids. The catalyst present within a transesterification process can be basic or acidic in nature. A base catalyzed transesterification process can, for example, use such catalysts as sodium methoxide, potassium hydroxide, sodium hydroxide and combinations thereof. In addition to a mono-alkyl ester biodiesel, a transesterification process yields glycerol, which can be used for cosmetic and pharmaceutical applications. The reaction stoichemitry of a conventional transesterification type process also results in by-products such as, for example, water, excess alcohol, light organics, heavies, free fatty acids and salts.
Primarily, the products from a transesterification process include fatty acid mono-alkyl esters and glycerol. The products also consist of a substantial amount of alcohol, which generally remains in excess due to, for example, the reaction stoichemitry. In order to obtain “fuel grade” biodiesel, fatty acid mono-alkyl esters produced through a transesterification process must be separated from glycerol and any reaction by-products such as heavies, alcohol, light organics, water, free fatty acids and salts. A fuel grade biodiesel is characterized by mono-alkyl esters that satisfy the specifications of the American Society for Testing and Materials (ASTM). Exemplary specifications set by ASTM for evaluation of fatty acid mono-alkyl ester biodiesel include ASTM D 6751.
Commonly, mono-alkyl esters from a transesterification type process are separated from glycerol via conventional means for liquid-liquid type separations such as, for example, a settling vessel. With both a batch or continuous type biodiesel process, such mechanical means of separation are based on the density and solubility differences between glycerol and mono-alkyl esters of fatty acids. In view of the substantial amount of excess alcohol present within fatty acid mono-alkyl esters and glycerol being separated, the physical dimensions of such mechanical separation means can be considerable. Settling vessels can also introduce extended delays in a biodiesel process.
To date, biodiesel processes are generally inefficient and involve both time consuming and maintenance intensive mechanical separations. Such conventional biodiesel processes also tend to suffer from marginal overall biodiesel yields. These shortcomings have impeded the progress of biodiesel as a viable source of alternative energy. The development of a biodiesel fuel as an alternative to standard petroleum-based diesel fuels requires a process that can overcome these shortcomings. Particularly, such a biodiesel process should be capable of continuously producing fuel grade fatty acid mono-alkyl esters from any conventional transesterification process. The process should also minimize the overall number of individual unit operations by providing for efficient and continuous separations.